This invention relates to a workpiece holder for supporting an annular workpiece by engagement between radially extending elements of the workpiece holder and the inner surface of the workpiece.
In one process of making large tires for earthmoving vehicles, layers of rubber and wire are applied to the surface of a toroidal shaped hollow sand core which remains within the tire carcass during the construction of the tire. The sand core is eventually dissolved as one of the final steps in making the tire. The sand core is somewhat fragile and care must be taken so as not to apply excessive force thereto during the manufacturing operations. Excessive force could crush or otherwise cause the sand core to fail.
In the latter steps of the manufacturing process, tire materials, such as uncured rubber, breaker plys and belts are applied to the tire carcass as it is being rotated about its axis. Heretofore, the carcass has been supported during such rotation by an apparatus having shoes which grip the outer cylindrical surface of the carcass, during application of materials to the sidewalls. Application of tire materials to the crown area of the tire involves other fixturing and a separate carcass handling step. This is both time consuming and adds to the manufacturing cost.
Thus, it is desirable to support the tire carcass by gripping the inner surface so that tire materials can be applied to both the sidewalls and the crown area. One of the problems with gripping the tire carcass from the inside is that the sand core is more prone to breakage if excessive force is applied to the inner surface or if the force is applied unequally. Preferably, the gripping force applied to the inner surface should be only slightly greater than that required to support the tire carcass. However, this creates another problem in that normally one support apparatus is used for several sizes of sand cores wherein the weight of the carcass varies thereby requiring different gripping forces for each of the several sizes.